Experimental results of gravity currents travelling over a fissured bed

Gravity currents are geophysical flows having large impact on human life and on the environment. The need to prevent and mitigate their negative consequences justifies the multitude of studies carried out to understand their physics. However little has been done concerning gravity currents travelling over porous bed, which is a frequent scenario in real life. Existing studies of gravity currents over porous substrates deal with lock-exchange gravity currents. In this paper we will present preliminary results of 23 experiments reproducing continuouslyfed brine currents initially developing over a flat bed and then running over a porous region of varying porosities. In correspondence of this region, 2D density maps are acquired by means of an image-analysis technique. Simultaneously an Acoustic Doppler Velocimetry Profiler allows to record a 3D instantaneous velocity profile. Results describing the effect of the bottom porosity on density and velocity profiles are presented. Surprisingly the current does not slow down while travelling over the porous substrate: the relaxation of the no-slip boundary condition together with fresh water incorporation from the bottom cavities allows high velocities in the near bed region. This modifies significantly velocity and density gradients at the lower current interface. Conversely, varying the bed porosity has no remarkable effect on the upper current interface.

Continuously-fed density currents over impermeable and porous substrates

Sara Venuleo

Gravity currents are buoyancy-driven flows having a significant impact on the environment and human life. They can be observed in a vast range of natural and anthropogenic scenarios, such as seawater and freshwater, the atmosphere, or industrial processes. Gravity currents transport sediments, nutrients, and oxygen in water bodies but they are also often dramatically involved in the spreading of contaminants and can represent a severe hazard to humans and infrastructures. The relevance of these phenomena has motivated extensive research on gravity currents such as the present study which addresses two main different research objectives. The first objective of this research concerns the modeling of gravity currents with Shallow Water Equations (SWE). Large Eddy Simulations data of continuously-fed gravity currents are used to discuss how the chosen definition of current depth affects the value of the coefficients appearing in the SW continuity and momentum equations due to the non-uniformity of the vertical distribution of flow variables. Moreover, the implications that different definitions of current depth have on research findings and entrainment parameterization are discussed. This analysis showed that coefficients appearing in the momentum equation might significantly (up to

30\%

) differ from one, depending on the chosen current depth definition. On the contrary, the coefficients appearing in the continuity equation were shown to be very close to one. The second main research objective was to study how the mean and turbulent structure of a density current changes when it propagates over a porous substrate and to characterize the volume exchanges occurring at its lower interface. Experiments were carried out in the Laboratory of Hydraulic Constructions at EPFL to reproduce continuously-fed density (saline) currents, which propagated first over a smooth horizontal bed and then over a porous substrate of limited length. Inflow discharge, initial excess density, substrate porosity, and downstream confinement of the substrate were systematically varied. An image analysis technique was developed to monitor the flow density evolution above the substrate and within its pores. Simultaneously, an Acoustic Doppler Velocimetry Profiler acquires a quasi-instantaneous 3D vertical velocity profile over the substrate. Experimental results showed that, after an initial phase during which the current sinks into the substrate, freshwater entrainment from the substrate pores starts. Consequently, a mixing layer gradually forms at the lower boundary of the current. Its presence affects the mean density, mean velocity, and turbulent field primarily in the near-bed region, where buoyancy contributes to turbulent kinetic energy generation together with shear. Moreover, at the porous substrate interface, buoyancy instabilities enhance the bed-normal momentum flux, increasing the bulk flow resistance. As a consequence, although the no-slip boundary condition results in lower friction losses, the shear velocity increases with the porosity of the substrate. In addition, experimental measurements allowed to quantify the current volume loss and the ambient-fluid entrainment from the substrate pores. The main drivers of these exchanges are discussed. The study of the entrainment of ambient-fluid from the bottom is fundamental to understand and model the resuspension of substances, such as nutrients and pollutants, present in the porous substrates.

EPFL2020

Continuously-fed gravity currents propagating over a finite porous substrate

Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss, Sara Venuleo

We present the results of laboratory investigations of continuously-fed density currents that propagate first over a smooth horizontal bed and then over a porous substrate of limited length. Inflow discharge, initial excess density, and substrate porosities are varied. Density measurements, acquired through an image analysis technique, are performed above the porous layer simultaneously with quasi-instantaneous vertical velocity profiles. After a first phase in which the current sinks into the substrate, freshwater entrainment from the bed begins and, gradually, a mixing layer forms at the interface between the surface flow and the porous bed. Shear-driven and Rayleigh-Taylor instabilities rule the dynamics of this mixing layer. The porous boundary effects are observed in the vertical distributions of both density and velocity, especially in the near-bed region. Here, larger flow velocities are recorded over porous substrates. We argue that these are due to the presence of a longitudinal pressure gradient, which in turn is a consequence of the current mass loss. Its presence over the porous substrate is proved by the current interface longitudinal slope. However, other effects of the presence of the porous substrate, such as the relaxation of the no-slip boundary condition and the bed-normal momentum exchange, also affect the velocity field. The turbulent structure changes significantly over the porous substrate: while streamwise turbulence decreases, shear and bed-normal Reynolds stresses increase in large part of the current depth. Buoyancy instabilities further enhance the bed-normal momentum flux and, in the near-bed region, contribute to turbulent kinetic energy generation together with shear. Published under license by AIP Publishing.

2019

Continuously-fed density currents over impermeable and porous substrates

Sara Venuleo

30\%

EPFL2020